The present invention concerns devices for measuring the torque transmitted by an engine shaft, for example a shaft of an aircraft turbomachine.
The present invention more particularly concerns a method for calibrating a torsion torquemeter preferably intended to be mounted in a helicopter turbomachine.
It is to be recalled that measurement of the rotational torque of a shaft is of particular importance in the field of helicopter engines since it generally provides one of the essential piloting data items taken into account by a pilot. Once the rotor of a helicopter has reached constant speed, the power supplied by the latter is exclusively dependent on torque.
Document FR 2 931 552 describes a torquemeter based on the measurement of torsional deformation of the shaft, this deformation in particular being a function of the transmitted torque.
The torsion torquemeter described in this document comprises:                a power shaft intended to transmit rotational torque about an axis, said power shaft being provided with a first wheel carrying a first and second series of angular reference points;        a reference shaft having a first end fastened to one end of the power shaft and a second end provided with a second wheel carrying third and fourth series of angular reference points, the angular reference points of the first and third series being parallel to one another, whilst the angular reference points of the second and fourth series are parallel to each other whilst being inclined relative to the reference points of the first and third series;        a measuring device to measure a first angular offset between two angular reference points respectively belonging to the first and third series of angular reference points, and a second angular offset between two angular reference points respectively belonging to the second and fourth series of angular reference points;        a calculation unit to determine a value of the torque output by the power shaft, based in particular on the first and second angular offsets measured by the measuring device.        
To calculate the torque supplied by the power shaft, the calculation unit must previously determine the temperature of the shaft using data provided by the measuring device. Once the temperature has been determined, the torque value is determined using a database previously stored in a memory of the calculation unit, this database containing torque values corresponding to several values of angle deformation for different temperatures.
The prior art torsion torquemeter therefore requires knowledge of the temperature of the power shaft, and of a behavioural model of the material of the power shaft.